You can either let Google's algo pick it for you or pick all videos from one partner. At this time Google is not allowing any video to be picked since it wants to make sure that the video does not have inappropriate content and complies with copyright law.

Thank you very much for your response AdSense Advisor (and everybody else). As the other people have commnented, every individual video I have tried to pick up hasn't been available via Adsense/Youtube.